A study of the respiratory mucous membrane was made in the turbinates of ferrets which had received repeated inoculations of influenza virus. There was no evidence that persistent immunity is related to the presence of a structural modification of the respiratory epithelium. In fact, the respiratory epithelium in fully immune animals differs histologically only in minor respects from that of the normal, untreated ferret. On the other hand, a functional difference exists between the normal and the previously infected animals as evidenced by a marked acceleration of the repair process in the latter. Serological studies at the time of reinfection, 4 months or more after the previous attack, indicate that a relation exists between the height of antibody titer and resistance. The degree of immunity is probably a product of serological immunity and the rate of tissue repair. The implications of these studies to the problem of influenza in man are discussed.